1. Field of the Invention
The present invention relates to a method of and an apparatus for correcting halftone-dot image data in printing devices including color scanners, image setters, CTP (Computer To Plate) devices, copying machines, and DDCP (Digital Data Color Proofing) devices, and a method of correcting halftone-dot threshold data to generate halftone-dot image data.
2. Description of the Related Art
There are known halftone-dot image output devices such as image setters for forming binary halftone-dot images on a sheet of photographic paper or film by producing black and non-black spots with a laser beam that is selectively turned on and off. It has been pointed out that such halftone-dot image output devices tend to generate a moire pattern in an outputted image due to an interference between their output resolution and the screen ruling (see Japanese laid-open patent publication No. 8-317212).
The output resolution is the resolution of an image output device and is represented by dpi (dots per inch), pixels/inch (equivalent to dpi), or pixels/mm. The screen ruling is defined as lines/inch (which may be converted into lines/mm) representing the number of halftone dots (also referred to as "halftone-dot cells") per unit length (1 inch). The screen ruling is represented by lpi (lines per inch), the number of lines, a screen frequency, or a halftone-dot frequency.
The moire pattern generated due to an interference between their output resolution and the screen ruling is a periodic pattern of halftone dots, i.e., a periodic interference pattern produced between the halftone-dot pitch and the scanning line pitch. The moire pattern appears as low-frequency noise which reduces the quality of the outputted image.
The inventor of the present invention has proposed processes for reducing the low-frequency noise as disclosed in Japanese laid-open patent publication No. 8-317218 (hereinafter referred to as "first process") and Japanese laid-open patent publication No. 9-200518 (hereinafter referred to as "second process").
According to the first process, the array of threshold values, i.e., the positions of threshold values, in a threshold template (also called halftone-dot threshold data) used for generating binary halftone-dot image data are arranged to equalize the number of pixels to be blackened (non-blackened) in the threshold template as much as possible for reducing the low-frequency noise. According to the second process, random numbers are added when the threshold values are arrayed in the threshold template according to the first process for further reducing the low-frequency noise.